This invention relates to electrostatic imaging devices, such as printers, which measure toner usage and which adjust operating voltages or the like to compensate for darkness shift of toner applied as toner is used from a depleting source of toner. A typical embodiment is a printer employing a replaceable toner cartridge from which toner is exhausted during printing.
The characteristics of an electrophotographic system can change over the usage of a given toner cartridge or other toner source. When certain characteristics change, there is a shift in print darkness over the life of use of the cartridge or other toner source as it depletes toward becoming empty. Factors which contribute to this change may include differences in toner with use (smaller particles tend to print earlier), photoconductor wear, and doctor blade wear. A gradual shift toward darker printing results from such changes.
U.S. Pat. No. 6,175,375, which is assigned to the same assignee to which this application is assigned, is to changing the electrophotographic operating points as a function of how much toner has been used from a toner source to compensate for the shift in print darkness resulting from usage. The operating points which can be changed to influence darkness are normally one or more voltage levels employed to charge the photoconductor, to charge a developer roller, or to transfer toner from the photoconductor to the paper or other media being imaged.
In order to carry out such darkness control, usage of a cartridge or other toner source must be available to the control mechanism of the imaging device. Accurate measurement for this purpose is important. It is desirable in certain applications not to store usage in the cartridge, but instead to use existing elements in the printer. Standard electronic printer control mechanisms include both temporary memory (random access memory or RAM) and some permanent memory (non-volatile memory or NVRAM), and a microprocessor or other data processing apparatus to operate on data and retrieve and store data in the RAM and NVRAM.
Also known is a toner cartridge from which the current level of toner is measured at the printer using the data processing apparatus of the printer. Specifically, U.S. Pat. No. 5,634,169, assigned to the assignee to which this application is assigned, discloses a torsion spring mounted drive to the toner stirring paddle which rotates in the hopper containing toner. When the toner reaches a certain level of depletion, the torsion spring yields less and less as the toner is depleted. The shaft to the toner paddle carries an encoder wheel, which may have multiple slots or other indicia for observation, but for the purpose of measuring toner, need only have spaced beginning and end slots. The time between observing the beginning slot and the end slot is related in a known amount to toner quantity, and pertinent factors are stored and the necessary data processing is carried out at the printer. Because of the varying postures of toner which occur in a hopper with stirring paddle, a running average is employed as the current toner-quantity measurement, a typical average being that of the last five paddle revolutions.
In accordance with this invention a toner container, such as a toner cartridge, in which toner load can be measured is employed with data processing apparatus in the corresponding imaging device to maintain printing darkness near a constant level during the use of the cartridge.
The factors defining changes in operating parameters with toner usage depend on the overall mechanism of the imaging device and are determined by testing and observation. These factors are stored in NVRAM of the imaging device. Since the imaging device typically has discrete darkness setting dictated by the print job or from the operating panel, such factors are required for each darkness setting.
As NVRAM can deteriorate with large amount of writing into the NVRAM, an objective is to limit the writing to the NVRAM in tracking toner usage. For this reason use data from measurement at the cartridge is entered into NVRAM only at predetermined levels. Between such levels, the RAM is used to store use data.
Another source of use data is the counting of pels printed (the pel being a single unit of a digital image). A typical digital image may be 1200 by 1200 dot per inch, so each pel is {fraction (1/1200)} inch on each side. Depending on the darkness setting different amounts of a printed pel may be conditioned to be toned, although it will appear only as a change in darkness as both the toner and the human eye tend to average the effect.
In the embodiments disclosed below, pel counting is employed to track usage between the predetermined levels observed at the cartridge and stored in the NVRAM. Such reliance on pel counting is more consistent with current usage over short terms, but may differ significantly from actual usage as graphics employing gray, for example, uses much less toner per pel than text. Accordingly, even when pel counting is used, periodic reliance on the measured toner amount at the cartridge is employed. The last amount from a measurement at the cartridge replaces the previous amount from pel counting after that last determination at the cartridge.
Also, when the imaging device is turned off or the cover opened, the toner amount measured at the cartridge is operated upon as the correct usage if it differs significantly from the amount in memory. This is important since the previous cartridge may have been replaced with a different cartridge.